Continuous measurement of a cyclist's power output is important because it adds greatly to the information available for analysing training activities and competition performance. This is particularly relevant for high performance competitors. Achieving higher power output for longer periods is one of the most important ways in which a competitive cyclist can improve their performance. Cyclists' training programmes are frequently based on maintaining specified power output for specified durations. Monitoring of power output during competition allows a cyclist to adjust their effort to make best use of their energy. If power output is recorded during an activity, it can be examined and analysed afterwards to identify areas needing improvement and to help form future training programmes.
There is a number of existing prior art systems for measuring a cyclist's power output on a bicycle when a bicycle is moving. Typically a prior art system displays instantaneous power to the cyclist on a unit mounted on the handlebars, and provides a facility to record the variation in power over a period of a few hours and download the information onto a computer later for analysis. There are four principal commercial systems available, each using different sensing techniques. Unlike measuring speed or pedalling cadence or heart rate, which can be done by observing and counting some activity without interfering with it, measuring force or power is very difficult to do without inserting a sensing element as a part of the power transmission path. The four existing power measurement systems each insert a sensor at a different point in the power transmission path between the cyclist's feet and the tyre contact with the road, as described below.
The first known system is “PowerTap” which uses strain gauges embedded in the hub of the rear wheel of the bicycle. This measures the mechanical strain as the rotational power is transferred from the rear gear cogs through the hub mechanism into the wheel itself. This system requires the use of a special rear wheel, built with the PowerTap hub at its centre. The use of a special wheel is a serious disadvantage, as competitive cyclists change wheels frequently. This system is described in U.S. Pat. No. 6,418,797.
A second system known as “SRM” uses strain gauges embedded in the right hand pedal crank that drives the chain. This measures the strain as the rotational power is transferred from the crank to the large gear ring driving the chain. This system requires the use of a special crank set containing the measurement sensors. The operation of this system is further described in http://www.srm.de/englisch/index.html.
The third system known in the industry as “Ergomo” uses strain gauges embedded in the axle that joins the two pedal cranks together through the bottom of the bicycle frame. It measures the strain as the axle twists slightly when the pedals are pressed by the cyclist. It measures the power from the left pedal only, and assumes that the power from the right pedal is exactly the same (which is hardly ever the case in practice). This system requires the use of a special axle and bearing assembly between the cranks. The Ergomo system is described in U.S. Pat. No. 6,356,847.
The “Polar” system is the only existing power measurement system that does not require a part of the bicycle to be replaced. It works by using sensors to monitor the tension in the chain, as disclosed in U.S. Pat. No. 6,356,848 The sensors are fitted close to the chain to monitor its speed and vibration. Installation requires measurement of the length and weight of the chain. It has a reputation for being difficult to install and calibrate, and for being less reliable than other systems.
All of the systems described above share a number of disadvantages. For example, installation of all these systems requires significant time, effort and expertise, and most cyclists will have to pay their local bike shop to do the job. The requirement to fit sensors and replacement parts to the bicycle means that the systems cannot be moved to another bicycle without significant time and effort. Most competitive cyclists own a number of bicycles, for different types of event, so this is a serious limitation. All but the Polar system require the use of special replacement parts, which limits the freedom of the cyclist to choose the parts that they want to use. None of the systems can accurately measure the power output from each foot, although they can infer it to some extent by relating variations in overall power output to the position of the pedals. This is not an accurate method, because it must assume that all the measured power comes from the foot on the downward stroke. It cannot take into account the power that the foot on the upward stroke may be adding (by pulling up) or subtracting (by pressing down). None of the above systems can measure “wasted” power, where a cyclist presses down on a pedal while it is on the up stroke. None of the above systems can measure how evenly and efficiently the cyclist is pressing on the pedal on the downward stroke.
PCT patent publication number WO2006121714 “Systems and methods of power output measurement” attempts to overcome the above mentioned problems. This PCT publication discloses a system for measuring the power output of a cyclist by placing sensors underneath the cyclist's shoe. A number of problems associated with this system include fitting the sensor between the cleat and shoe increases the distance from the cyclist's foot to the axle of the pedal, which reduces cycling efficiency. Variations in the force used to screw the cleat to the shoe will cause unknown force on the sensor, leading to an unknown “zero” point and calibration errors. The system takes no account of the need to measure the pedalling rate (“cadence”) and the angular position of the crank at each instant. Rotational power at any instant cannot be calculated without knowing the applied force, the speed of rotation and the angular position of the crank thus leading to inaccurate data. The sensor and its connector are under the shoe, and therefore are exposed to damage during use.
PCT patent publication number WO2008/058164, assigned to Quarq, discloses a system that operates very similarly to the SRM system (described above). The system requires use of a special crank set with strain gauges embedded in the right hand side of the crank. The system also discloses the use of an accelerometer for certain measurements, when mounted on the crank assembly and requires a magnet on the bicycle. The system disclosed in Quarq is complex to implement as there is different crank sets for different manufacturers.
US patent publication number US 20070245835, assigned to Microsport, describes a system using measurements from a flexible force sensor inside a cyclist's shoe. The system measures only the compression force normal to the plane of the shoe and pedal. The system does not include any means to measure the direction of the force being applied through the pedals to the cranks, and uses pre-calculated estimates of direction based on assumptions of standard cycling styles.
There is therefore a need to provide a device, system and method to overcome the above mentioned problems. An object of the invention is to provide a device and method to measure, display and record the power output of a cyclist accurately and more effectively than current solutions on the market.